1. Field of the Invention
The present invention relates to centrifugation, and more particularly to centrifuge rotors and lids.
2. Description of Related Art
FIG. 1 shows the sectional view of a prior art "fixed angle" centrifuge rotor 10. The rotor has an axially symmetric body provided with several cavities 12 about the rotor axis. Each cavity is sized and shaped to receive a centrifuge tube 13 containing the sample to be centrifuged. A lid 14 is provided to cover the opening 15 of the rotor to contain aerosol. A lid knob 16 having a threaded end is provided for bolting the lid 14 on the rotor 10. A spindle knob 18 having a shaft is provided for bolting the rotor 10 to the spindle 20 of a centrifuge drive. Several O-ring seals 21-24 are provided for sealing adjoining structures as shown.
Prior to centrifugation, the rotor is positioned on a laboratory bench and centrifuge tubes 13 containing samples are loaded into the rotor cavities 12. Typically, there may be locking and sealing mechanisms for each cavity (not shown). The lid 14 is placed to cover the rotor opening 15 and the lid knob 16 is hand tightened to secure the lid 14 on the rotor 10. The rotor is then carried to the centrifuge and lowered onto the spindle 20. The spindle knob 18 is hand tightened to secure the rotor hub against the spindle.
For some applications, the rotor may be used in a centrifuge which draws a vacuum in the centrifuge chamber. In bioresearch applications, one approach to centrifugation is to suspend the sample in a cesium chloride density gradient forming solution, a technique well known in the art. Upon centrifugation, the sample components separate into regions or bands parallel to the axis of rotation, and at distances from the axis depending on their relative densities.
When centrifugation has been completed and the rotor comes to rest in the centrifuge, air is let into the vacuum chamber when the door to the chamber is opened. The rotor 10 is lifted from the spindle 20 and carried to the laboratory bench where the lid 14 and the centrifuge tubes 13 are removed from the rotor cavities. The handling of the rotor 10 at the end of centrifugation requires special attention. The rotor must be handled carefully in a way such that the separated sample component bands are not disturbed by vibration. Otherwise, mixing of sample components occurs which defeats the principal of centrifugation. In the past, the rotor is handled by holding the bottom and sides of the rotor using both hands of the user. It becomes physically demanding on the user to move the heavy rotor this way, especially when lifting rotors that are over 10 kilograms.
When removing the rotor lid 14 after centrifugation, even though the lid knob 16 had only been hand tightened prior to centrifugation, it has been found that it was necessary to use a tool to loosen the lid knob 16 after centrifugation. This is caused in part by atmospheric pressure on the lid 14. It was experienced that when the rotor is subject to prolong period of vacuum during centrifugation, air in the space under the lid 14 inevitably escapes through the O-ring seals, due to reduction of sealing effect under high centrifugal field. When the outside of the rotor is again at atmospheric condition at the end of centrifugation, it was experienced that a partial vacuum is retained under the lid. The pressure difference gives rise to a large force on the upper surface of the lid which presses the lid tightly against the rotor. When the lid knob is turned in an attempt to lift the lid, this force is overpowering against the hand turning torque capable of a typical user. The user had to use a tool (e.g. a wrench) to provide the additional torque required. Another cause for the increased tightening of the lid knob after centrifugation arises from the increased friction at the O-ring seals and the threads between the lid knob and rotor body as affected by centrifugation (parts expand during centrifugation and contract after centrifugation at different rates).